sending multiple respone from nodejs using python-shell - node.js

Question

I am trying to execute few python script inside nodejs. The code is shown below. What I am trying to do is executing different python script inside a for loop one by one. and send the json response to client as soon as one script gets over.
var PythonShell = require('python-shell');
var express = require('express'), app = express();
app.get('/', function (req, res) {
res.setHeader('Content-Type', 'text/html');
pl_list=["test", "test2"]
for (var i=0; i<= pl_list.length-1; i++) {
output="";
var pyshell = new PythonShell('./'+pl_list[i]+'.py')
pyshell.on('message', function (message)
{console.log(message);output+=message;});
pyshell.end(function (err) {
if (err){
console.log('error occured ---- '+err);
}
else{
console.log('update finished');
res.write(JSON.stringify({"finsihed":true, "product_line":pl_list[i]}));
}
});
}
//res.end()
});
app.listen(5000, function () {
console.log('The web server is running. Please open http://localhost:5000/ in your browser.');
});
unfortunately I am getting the response as {"finsihed":true} actual output must be
{"finsihed":true, "product_line":"test"}{"finsihed":true, "product_line":"test2"}
can anybody tell me what I am doing wrong here. Thanks in advance!

Answer 1

The execution of your python scripts is asynchronous, so when you write the response to the client with this line, the value of i changed:
res.write(JSON.stringify({"finsihed":true, "product_line":pl_list[i]})
Just display the value of i with console.log before the above line and you will see that i equals 2 twice (due to the increment of your for-loop). And because pl_list[i] is undefined, the serialization of a JSON object removes the attribute "product_line".
If you want to "save" the value of i, you have to learn what closure is.
This code should work:
var PythonShell = require('python-shell');
var express = require('express'),
app = express();
app.get('/', function (req, res) {
res.setHeader('Content-Type', 'text/html');
var nbFinishedScripts = 0;
pl_list = ["test", "test2"]
for (var i = 0; i <= pl_list.length - 1; i++) {
output = "";
var pyshell = new PythonShell('./' + pl_list[i] + '.py')
pyshell.on('message', function (message)
{
console.log(message);
output += message;
});
// closure
(function (i) {
return function () {
pyshell.end(function (err) {
if (err) {
console.log('error occured ---- ' + err);
} else {
console.log('update finished');
res.write(JSON.stringify({
"finsihed": true,
"product_line": pl_list[i]
}));
}
nbFinishedScripts++;
// end the reponse when the number of finished scripts is equal to the number of scripts
if (nbFinishedScripts === pl_list.length) {
res.end();
}
});
};
})(i)(); // immediately invoke the function
}
});
app.listen(5000, function () {
console.log('The web server is running. Please open http://localhost:5000/ in your browser.');
});
Edit code:
var PythonShell = require('python-shell');
var express = require('express'),
app = express();
var executePythonScript = function (script) {
return new Promise(function (resolve, reject) {
var pyshell = new PythonShell('./' + script + '.py');
pyshell.end(function (err) {
if (err) {
reject(err);
} else {
resolve(script);
}
});
});
};
app.get('/', function (req, res) {
res.setHeader('Content-Type', 'text/html');
var pl_list = ["test", "test2"];
Promise
.all(pl_list.map(executePythonScript))
.then(function (scripts) {
scripts.forEach(function (script) {
res.write(JSON.stringify({
finsihed: true,
product_line: script
}));
});
res.end();
})
.catch(function (err) {
res.end();
});
});
app.listen(5000, function () {
console.log('The web server is running. Please open http://localhost:5000/ in your browser.');
});